If I use Microsoft's file sharing does it automatically use CIFS?

In other words, if I have two Windows 2000 work-stations that are
sharing files with each other...
are they using CIFS, or some other protocol?
For that matter, any of the workstations could be Windows XP or Windows

Or, does one of the stations need to be a Winows Server running SMB (or
CIFS) server?